Underpants-type disposable diapers include an outer sheet having a front body part and a back body part, and an inner body containing an absorbent body and fixed to an internal surface of the outer sheet, and the front and back body parts of the outer sheet are joined together at both side portions to form a waist opening and a pair of right and left leg openings.
In some underpants-type disposable diapers, various elastic members are fixed in a stretched state to the outer sheet for improvement of a fit to the body of a wearer (refer to Patent Literatures 1 to 7, for example). In particular, underpants-type disposable diapers including waist-portion elastic members, waist-around elastic members, and curved elastic members (represented by the members disclosed in Patent Literature 6) have a relatively high fit to the body of a wearer. The waist-portion elastic members are laterally arranged in plural and in parallel at vertical intervals therebetween at an edge portion of the waist opening. The waist-around elastic members are laterally arranged at vertical intervals therebetween in a section at a longitudinally intermediate portion side with respect to the waist-portion elastic members. The curved elastic members extend while being curved from both side portions of at least one of the front and back body parts toward the other body part with increasing proximity to a laterally intermediate position.
However, conventional general underpants-type disposable diapers have a problem of causing a puffy and discomfort feeling at a crotch portion, unlike general underpants. The inventors of the present invention have studied and revealed that one cause of this problem resides in that an absorbent body lifts from a part of the body of a wearer ranging from the intergluteal cleft to the crotch, that is, the absorbent body does not sufficiently fit the part.
To solve the foregoing problem and improve a fit to the intergluteal cleft, there have been suggested various modes in which, for example, back-side curved elastic members are arranged at a crotch traversing portion so as to project in a convex shape toward the waist opening (refer to Patent Literatures 8 and 9, for example). However, such diapers have an internal surface formed so as to fit the intergluteal cleft but are hard to maintain the fitted state, and thus are prone to provide an insufficient fit to the part from the intergluteal cleft to the crotch. In addition, at manufacture of such diapers, elastic members need to be changed in orientation at a steep angle before attachment, which makes it difficult to manufacture the diapers in high-speed lines.